Silicon dioxide films have been used for some time in the fabrication of integrated circuits (IC) for semiconductor device manufacturing. There are many examples of the preparation of such thin films of SiO2 in the open and patent literature. See, for example, the publications of the Schumacher Group, Air Products and Chemicals, Inc., e.g. User's Guide For: Glass Deposition with TEOS1, and Extrema® TEOS (Tetraethyl Orthosilicate) Product Data Sheet2. See also, Modeling of Low-Pressure Deposition of SiO2 by Decomposition of TEOS3, and The Deposition of Silicon Dioxide Films at Reduced Pressure4. There are numerous journal articles that review various CVD techniques for the deposition of SiO2 and the properties of thin films deposited using such techniques5-9.
Early SiO2 films were deposited by CVD oxidation of silane (SiH4). New source materials were needed in order to maintain good step coverage as sub-micron patterned electronic devices were developed. Films deposited from tetraethylorthosilcate (TEOS) show superior step coverage properties compared to SiH47. TEOS is considered an industry standard source for the CVD preparation of SiO2. TEOS is a volatile liquid, providing for efficient vapor delivery and general ease of handling. It is nonpyrophoric, and therefore, provides a significant safety advantage over silane. It produces dielectric films with excellent electrical and mechanical properties suitable for many device manufacturing applications.
The chemical 1,3,5,7-Tetramethylcyclotetrasiloxane (such as TOMCATS® siloxane available from Schumacher of Carlsbad, Calif.) is under development as a new source material for the CVD preparation of SiO2 glass10-11. TOMCATS type siloxane is a high purity volatile liquid precursor chemical that is specifically designed to satisfy the critical demands of the semiconductor device manufacturing industry. Like TEOS, TOMCATS type siloxane can be used for the chemical vapor deposition of glasses and doped glasses for various dielectric film applications such as trench fill, interlevel dielectric, gate and thick oxide2. It provides similar safety advantages because of its non-pyrophoric and noncorrosive nature. The normal boiling points of TOMCATS type siloxane and TEOS are 135° C. and 168° C., respectively. The higher volatility of TOMCATS type siloxane allows it to be delivered at lower temperature or with higher efficiency at comparable temperature. Its deposition rate is 10 times that of TEOS at 600° C., with a deposition efficiency 3 times that of TEOS2. It is superior to silane and similar to TEOS in the conformality and step coverage of the resulting films11-12.
In general, SiO2 films deposited from TOMCATS type siloxane exhibit excellent mechanical and electrical properties. The films are dense with low carbon content and refractive index values comparable to thermal oxide. TOMCATS type siloxane is effective for low-pressure chemical vapor deposition (LPCVD) and as a liquid injection source for plasma enhanced chemical vapor deposition (PECVD). The later method utilizes plasmas rather than thermal energy to promote chemical reactions. TOMCATS type siloxane PECVD is typically run at lower temperature than LPCVD (400° C. vs. 500-600° C.).
Despite these advantages, TOMCATS type siloxane has experienced limited acceptance as a CVD source for the manufacturing of semiconductor devices. One disadvantage of TOMCATS type siloxane is its instability with respect to polymerization13 when exposed to certain chemicals or process conditions. This results in a lower volatility liquid or gel that creates CVD process handling issues. TOMCATS type siloxane polymerization is catalyzed by acid or base. It has been observed in the present invention experimentally to be particularly sensitive to exposure to bases (see Examples 9-11 below).
Prolonged heating of TOMCATS type siloxane (Example 1) has also been shown experimentally in the present invention to promote polymerization. The degree of polymerization can be very minor, accounting for only several tenths of a percent. Under more severe conditions of prolonged exposure to elevated temperature or to certain acids or bases, substantial polymerization will occur, resulting in a highly viscous liquid or gel containing over 10% by weight of oligomeric or polymeric material.
Several references in the prior art relate to the stabilization of siloxane. Hirabayashi et al.14 teach the use of a triazine or sulfide “control agent” to stabilize a mixture comprising an aliphatic unsaturated group, containing an organopolysiloxane compound, such as TOMCATS type siloxane, and a platinum group catalyst. Those inventors teach the use of the triazine or sulfide agent to give a mixture that is stable and resistant to premature gelation at room temperature and thus providing extended storage stability.
Lutz et al.15 disclose the use of di- and trihydrocarbylphosphines which act as curing inhibitors for compositions comprising: (1) alkenyl radicals; (2) compounds containing silicon-bonded hydrogen atoms (e.g., TOMCATS type siloxane); and (3) a platinum group metal catalyst. Lutz et al. claim that the inhibitor functions by complexing with the platinum catalyst rendering it inactive for subsequent curing.
In a similar patent, Chalk16 teaches the use of acrylonitrile type compounds that reduce the activity of the platinum catalyst deterring the copolymerization of various mixtures of polysiloxanes.
Berger et al.17 propose the use of an ethylenically unsaturated isocyanurate which functions in a like manner to deactivate the Pt catalyst rendering a curable organopolysiloxane composition stable to premature gelation.
Endo et al.18 teach the stabilization of cyclosiloxanes, such as TOMCATS type siloxane through the use of 1 to 20 weight % of polymethylpolysiloxanes, such as 1,1,1,3,5,5,5-heptamethyltrisiloxane.
The patent references cited all teach the use of various agents that in one manner or another inhibit the polymerization or co-polymerization of polysiloxanes for various applications in the silicon rubber industry. None of them specify or suggest applications as polymerization inhibitors for CVD sources in the semiconductor device manufacturing industry.